iPhone 4: Of Course Glass Breaks

Some people have found that the glass back on the new iPhone 4 can
break when dropped. This has led to the false idea that using glass was
a stupid idea on Apple's part.

This ignores two simple facts: The first is that the display has
been glass for years without any big complaints, and the second is that
the right glass can be incredibly strong.

Glass Pros and Cons

Glass, like any ceramic, is very strong in
compression. This means that it can handle weight pushing on it.

Glass is weak in tension and shearing (being pulled or bent). What
usually breaks glass is bending or being struck by a pointed object.
Bending causes the inside to go into compression, which is just fine,
but the outside is in tension, which is bad. Glass almost explodes into
pieces when it is bent. Pointed objects hitting glass also cause it to
break, because all the force is concentrated into a small area.

Glass is tempered or toughed
by cooling it in a way to create stress on the surface. Stress is
usually bad: No one likes stress at work, and stress on an object can
break it. But with glass, the stress is compressing the glass. Remember
that glass is stronger in compression. With tempered glass, when
someone bends it, the force has to overcome the built-in stress before
it will break.

I got my degree in Material Science, so I've spent a few years
learning how to make materials stronger and how they break. Using glass
is fine for all sorts of things - you just need to know the
limitations. The right kind of glass at the right thickness can stop a
bullet. Windows on submarines withstand thousands of tons of pressure
safely. Sure, glass can and does break, but only when you use it
incorrectly.

For Instance...

One of the guys at Gizmodo
dropped his iPhone on the first day. One small drop (three to four
feet) onto the curb, and the back was broken.

Did he have a case to protect it?

No.

Did he use one of those bumper things that Apple sells?

No.

If you look at the picture on Gizmodo, the crack seems to start from
one spot on the edge. There is a reason for this. Tempered glass, which
I assume Apple is using some variation of,* is extremely sensitive to
damage at the edge.

One place I worked used tempered glass on work tablets for cutting
materials and building parts. The glass could take a lot of abuse
without wearing out, perfect for the kind of work we did. But if you
accidentally tapped the edge with a box knife, the whole thing would
shatter into a million pieces. This happened a couple of times until we
protected the edges.

Glass Is Not a Stupid Choice

There is a special type of glass that really shows both how strong
and weak glass can be, Prince Rupert's Drops.
These are made by dropping molten glass into water. They make a
teardrop shape. The bottom is strong enough to take a hammer blow, but
if you bend the tip it will explode into small bits.

Apple's choice of glass is not stupid, but iPhone 4 owner need to do
a couple of smart things - either never drop the phone on its edge or
protect the edges. At our house, the first thing we do with a new
iPhone is buy a case for it. iPhones are not cheap to replace, so
buying a nice case only makes sense - but if you want to trust yourself
to never drop your iPhone, be my guest.

Apple expects most people to buy a case. If you are too cheap to buy
a case. At the very least, put a rubber band or some thick tape around
the edges. Apple sells bumpers and cases
for a reason.

Apple's $29 Solution

If everything I say is true, why didn't Apple either build shock
protection around the glass or give bumpers away with the iPhone?

iPhone 4 with Bumper

Since Steve Jobs doesn't call me to explain things, I'll have to
make a guess.

First is the fact that the look of an iPhone is strikingly different
with bumpers. Sometimes Apple picks design and then tries to get
function to work around it.

Second, Apple knows that most people (probably 90% or better) buy
some type of case; Apple is banking on us taking care of our new
toy device.

Finally, nothing is free. Instead of adding a "free" bumper that
most people would ignore when they bought a case, Apple saves money on
each iPhone made and sells bumpers separately.

Why is Apple charging so much for a big rubber band? One reason is
that it increases the chance that people will buy a third-party case. I
mean, who would spend $29 for a fancy rubber band when you can get a
nice case instead? Another reason: Why not make money doing selling
iPhone protection? This isn't like the iTunes Store, which was created
to drive hardware sales. Accessories are sold to make extra money.

If you have an iPhone 4 and are waiting to find the perfect case, be
careful with it until you've got some protection for it.

* Editor's note: iFixit speculates
that it is made of Corning Gorilla Glass, although Apple makes no such
claim. If Apple were using Gorilla Glass, we suspect that they would be
open about it, since it's considered a selling point on smartphones
that use it.